1. Field of the Invention
The present invention relates to a self-contained wood-burning furnace for insertion in an existing conventional fireplace for heating room air and recirculating such air back into the room.
2. Description of the Prior Art
Conventional fireplaces are notoriously inefficient room heaters because when used they draw warm room air into the fireplace and up the chimney.
Glass doors are commonly used to close fireplace openings to reduce the loss of warm room air through the fireplace without reducing visibility of the fire itself. Such glass doors are effective so long as there is another source of heat for the room, but they prevent the fire from radiating heat into the room.
Others have sought to solve the problem of converting a conventional fireplace to an efficient room air heater but without entire success. Many such fireplace heaters must be built into the fireplace itself, thereby requiring construction of the heater at the same time that the fireplace is built. Heaters of this type are represented by U.S. Pat. Nos. 1,706,768 and 2,165,661.
So-called fireplace heat extractors have recently become popular. Typically they comprise a system of heat exchange tubes and a blower which draws room air into the tubes and forces it back into the room. The tubes are usually placed along the back of the fireplace. Such heat extractors generally have a low air-moving capacity and therefore usually are ineffective to heat large spaces. Repesentative heat extractors are shown in U.S. Pat. Nos. 2,052,643; 2,161,723; 2,702,030; 3,240,206; 3,635,211; and 3,901,212.
The closest known prior art is my own prior U.S. Pat. No. 4,015,581, issued Apr. 5, 1977, and the prior art references cited therein. My aforementioned prior patent discloses a self-contained fireplace heater or furnace for insertion within a conventional fireplace. The heater unit includes a metal firebox within a surrounding metal jacket. Room air is drawn into side inlet openings of the jacket, then warmed by the firebox walls serving as heat exchanger surfaces, and then expelled back into the room through an upper portion of the jacket, all by natural convection. Although this heater is satisfactory to maintain warm temperatures in a room, it does require a considerable length of time to heat a cold room to a desired temperature, particularly a large room, because of the low rate of air movement through the heater by natural convection.
Others have suggested using fans to provide forced air circulation through firebox-type heaters. See, for example, U.S. Pat. Nos. 2,642,859 and 2,743,720. However, this does not entirely solve the problem because of the tendency of such fans to move air through the heater and back into the room before it can be adequately warmed because of the limited heat exchange surface area in the heater.